As a technique for realising high recording density of an optical recording medium, such a technique has been devised which exploits evanescent light to enable recording and/or reproduction at an extremely small recording pit not larger than the diffraction threshold. In recording and/or reproducing an optical disc using the evanescent light, the incident light beam to a lens is converged on an end face of the lens such that its major portion undergoes total reflection on the lens end face. If the distance between the lens end face and the optical recording medium is made sufficiently narrow, a portion of the evanescent light is coupled with the optical recording medium so as to be taken out to outside the lens to enable recording and/or reproduction exploiting the evanescent light.
The distance for which the evanescent light can be coupled in this manner in air is on the order of 100 nm if the numerical aperture NA of the lens is 1.5. Therefore, if recording and/or reproduction of an optical recording medium is to be performed using the evanescent light, the distance between the lens end face and the optical recording medium needs to be maintained at a level not larger than approximately 100 nm. This can be realized using the flying head technique used e.g., in a magnetic disc.
That is, if, with the use of the flying head technique used in a magnetic disc, the distance between the lens end face and the optical recording medium is maintained at approximately 100 nm or less, a portion of the evanescent light is coupled with the optical recording medium to enable recording and/or reproduction at an extremely small recording pit not larger than the limit of diffraction.
The technique of exploiting a catadioptric lens for recording and/or reproduction exploiting this evanescent light has been proposed by C. W. Lee of Samson Electronics Inc. at an optical data storage meeting in USA in May 1998.
However, analysis of lens data of the catadioptric lens proposed by C. W. Lee reveals that correction of the coma aberration is not optimum, such that use of this catadioptric lens raises the following problem:
That is, if the evanescent light is to be used, the incident light beam needs to be converged with a small spot diameter on the lens end face. However, since the coma aberration can be corrected only incompletely with the catadioptric lens, such that, if the angle of incidence of the light beam exceeds .+-.1.degree., due to an error in the mounting angle of the catadioptric lens, the coma aberration is produced outstandingly, such that it becomes impossible to converge the incident light beam on the lens end face with a small spot diameter.
Moreover, in a near-field optical system, exploiting the evanescent light, it may be estimated that the effect of the coma aberration presents itself more significantly than if a light beam is converged in a far-field optical system with a small spot diameter. Therefore, if assumed that the catadioptric lens is used in the near-field optical system, it may be premeditated that, when mounting the catadioptric lens, the error in the mounting angle thereof needs to be appreciably smaller than .+-.1.degree..
That is, since the correction of the coma aberration is incomplete in the catadioptric lens, extremely high precision is required in mounting the catadioptric lens. In particular, it is extremely difficult to realize the mounting at a precision which permits use of the catadioptric lens in the near-field optical system. If this could be achieved, the manufacturing cost is necessarily prohibitive.